In Eucaryotes, normal development, as opposed to neoplastic growth, entails the orderly differentiation of cells within a narrowly defined temporal and spatial framework. Homothallism, inherited switch of the mating type locus in bakers yeast, Saccharomyces cerevisiae, is exploited as a model system to study the mechanisms involved in differentiation of cell types. Yeast cell types are controlled by two allelic forms of the mating type locus (MAT) a and alpha. Homothallic strains can change these cell types by genetic alterations at MAT in a non-random fashion as often as every cell generation. We have been working on the mechanism of mating type switching. The results obtained allow us to conclude that yeast contains silent mating type information at loci HMa and HM alpha which are loosely linked to MAT. We have shown that during the switching process a replica of the silent information replaces the information existing at the mating type locus. We have also obtained mutants which allow the expression of the silent information in situ at HMa and HM alpha. Also, we are isolating and characterizing mutants of the MAT locus to investigate as to how this locus controls the cell type.